1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that is capable of smooth and efficient toner conveyance.
2. Discussion of the Background
Related-art image forming apparatus, such as copying machines, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, and facsimile functions, include a developing device.
FIG. 1 is a typical developing device used in an image forming apparatus. As shown in FIG. 1, the developing device includes a developing roller 103, a supply roller 104, a blade 102, a toner hopper unit 107 and a toner agitator 105. The supply roller 104 is a rotary body and has an outer circumferential sponge layer. The supply roller 104 is rotated in the same direction as the developing roller 103. While rotating, the supply roller 104 captures toner with the sponge layer to supply it to the developing roller 103.
The blade 102 is formed of a plate spring made of metal. When a top portion of the blade is pressed against the surface of the developing roller 103, a thin toner layer is spread uniformly over the surface of the developing roller 103.
The toner agitator 105 is provided in the toner hopper unit 107. By rotating the toner agitator 105, the toner agitator 105 agitates the toner stored in the toner hopper unit 107. The developing roller 103 is a rotary body and has an outer circumferential rubber layer.
The developing roller 103 contacts the surface of the photoconductor 101 that is an image carrier. The thin toner film layer uniformly formed on the surface of the developing roller 103 by the blade 102 is transferred onto the surface of the photoconductor 101 to form an toner image.
Toner on the surface of the photoconductor 101 is transferred onto a sheet of paper or a transfer belt, not shown. However, residual toner that is not transferred but remains on the photoconductor 101 is scraped off by a cleaning blade 108 that is elastic and is pressed against the photoconductor 101. These processes are repeated, and the toner scraped off by the cleaning blade 108 is accumulated in a cleaning unit 109 for the photoconductor 101.
Unused toner accumulated in the cleaning unit 109 for the photoconductor 101 may be reused by returning it to the developing unit, or not reused but just collected.
Over time, paper particles and duct may adhere to the surface of the photoconductor 101. Further, in a color image forming apparatus that uses a plurality of color toners, toner of other colors may be adhered to the surface of the photoconductor 101. These particles are mixed with the toner scraped off from the photoconductor 101, and if returned to the developing unit, the recycled toner may contain more impurities than fresh toner. As a result, it is difficult to maintain high image quality. Consequently, in most image forming apparatuses other than monochrome printers, the toner scraped off from the photoconductor 101 is not reused but is just collected.
When the toner is collected, the toner accumulates in the cleaning unit 109 for the photoconductor 101. Accordingly, the cleaning unit 109 is designed to have sufficient volume to store the accumulated toner.
By contrast, when the toner is reused, a conveyance mechanism is necessary to convey the collected toner from the cleaning unit to the developing unit. In both cases, it is necessary to convey toner from the cleaning unit to the other unit.
To convey toner (waste toner), a conveyance belt as shown in FIG. 2 may be used. More specifically, a conveyance belt 111 is provided in a conveyance path 112 to tilted at a predetermined angle with respect to the vertical. The conveyance belt 111 is extended around a driving shaft 113 provided at an upper part of the conveyance path 112 and a roller 114 provided at a lower part of the conveyance path 112. Projections 116 are formed on the driving shaft 113 that engages an engagement groove, not shown, formed in both sides of the conveyance belt 111 to transfer the driving force of the driving shaft 113 to the conveyance belt 111.
When the driving shaft 113 is driven, the conveyance belt 111 is moved in a direction shown by arrow A in FIG. 2. Further, projections and depressions 115 are formed on an outer surface of the conveyance belt 111. Accordingly, when the conveyance belt 111 is moved, the toner T is conveyed from the lower part to the upper part of the conveyance path 112. The toner T scraped off from the photoconductor 101 is supplied to a lower position of the conveyance belt 111 by, for example, a lower conveyance screw. Further, an upper conveyance screw is juncturally provided on the driving shaft 113. The toner T is conveyed to a waste toner collection unit by the upper conveyance screw. It is to be noted that although in the example shown in FIG. 2 the projections and depressions 115 are represented for only a part of the conveyance belt 111, the projections and depressions 115 are actually formed along the entire surface of the conveyance belt 111.
Since the driving shaft 113 applies pressure on the conveyance belt 111, the tension on the conveyance belt 111 is relatively high at an upstream end of the driving shaft 113 and relatively low at a downstream end of the driving shaft 113. Accordingly, the conveyance belt 111 may sag, with the possibility that the conveyance belt 111 gets caught by and entangled with the driving shaft 113.
To avoid such a problem, a rib member 122 may be provided along a line connecting the axes of the roller 114 with a shaft center of the driving shaft 113 as shown in FIG. 2. In this case, a gap between the rib member 122 and the driving shaft 113 is set as small as possible so that the conveyance belt 111 is not caught by the driving shaft 113 knocking against the rib member 122.
In the toner conveyance device shown in FIG. 2, it is desired that the toner conveyed be scraped off completely at a portion of the conveyance belt 111 wrapped around the driving shaft 113. However, even if that removal is accomplished, the toner may still build up on the projections and depressions 115 of the conveyance belt 111. As a result, it is difficult to completely remove the toner adhered even with a scraper like that described in Japanese Patent Laid-Open Application Publication No. 2008-175956-A, for reasons that are described below.
In addition, when the conveyance belt 111 on which the toner is adhered is moved, the toner may get onto the conveyance belt 111 through the engagement grooves that transfer the rotational drive force. Consequently, toner is constantly flying in and around the toner conveyance path because the conveyance belt 111 on which the toner is adhered is moved and the toner is conveyed by the conveyance belt 111. Accordingly, the toner may gradually accumulate at areas outside that portion of the toner conveyance path through which the conveyance belt 111 moves, which is undesirable.
Further, the toner may accumulate in a space between the rib member 122 for preventing the circulating conveyance belt 111 from being caught and the conveyance belt 111. Thereafter, the toner may be moved to the roller 114 provided downstream in the circulating direction by some impact or friction generated by the circulation of the conveyance belt 111. In such case, since the conveyance belt 111 is circulated, the toner may be captured in a space between the roller 114 and the conveyance belt 111 to shift outward by an amount equal to the thickness of the toner T captured. Since the conveyance belt 111 is traveling with a predetermined but generally relatively narrow gap between it and a wall surface of components that define the toner conveyance path, any outward displacement of the conveyance belt 111 entails a risk that the conveyance belt 111 may get hung up on the wall surface of the toner conveyance path.
The above-described situation is not improved with the removal of the intermediate rib member. FIG. 3 is a toner conveyance device lacking the rib member 122. In such toner conveyance device, the toner is flying about in the vicinity of the wall surface of the toner conveyance path and gradually piles up against a wall 127 perpendicular to the moving direction of the conveyance belt 111 (that is, on a plane parallel to the sheet of paper on which FIG. 3 is drawn).
Moreover, normally the toner does not collapse while the conveyance belt 111 moving. However, when the toner is subject to some impact, the toner does not collapse and move. In such case, similarly to the toner conveyance device having a rib member 122, the toner collapsed and moved is captured in the space between the roller 114 and the conveyance belt 111, shifting the traveling position of the conveyance belt 111 outward by an amount equal to the amount of the toner T thus accumulated. Consequently, the conveyance belt 111 may get hung up on the wall surface as described above.